The upslope, shoreward transport of colder, nutrient-rich waters from below the ocean thermocline is thought to be critical to coastal ecosystems such as coral reefs. Internal bores, or boluses, generated by the breaking of shoaling internal waves on the slope provide a mechanism for this cross-shelf transport of dense water. Indeed, observations confirm that as internal waves propagate into shallow water, the waves steepen, which can lead to breaking and, at times, the generation of turbulent boluses of dense fluid which continue to travel inshore, mixing with ambient surface water. This mechanism is thought to be a significant mechanism for the transfer of nutrients, sediment, and larvae between deep waters and the shelf environment. Furthermore, bolus-driven transport has the potential to transport contaminants from offshore sewage outfalls at depth into the near-shore environment, posing risks for human health through fishing and direct exposure at beaches. However, questions still remain regarding internal bore propagation and transport, including what conditions cause the formation of internal bores. Thus, this project will investigate the internal bore formation and mass transport processes using laboratory experiments.
To date, the knowledge of internal bore formation, propagation, and associated mass transport is fairly limited. The overall goals of the research, therefore, are to (1) quantitatively determine the physical conditions which control the development of internal bores from shoaling internal waves, and develop models and parameterizations for this, and (2) relate initial flow parameters to the mass transport and fluid entrainment of internal bores. A series of laboratory experiments in the internal wave facility of the Stanford Environmental Fluid Mechanics Laboratory will study the shoaling of internal waves on uniform slope/shelf topography in a two-layered system. Quantitative flow visualization techniques, including Planar Laser-Induced Fluorescence (PLIF) and Particle Image Velocimetry (PIV) will allow a detailed study of the effect of variations in the initial flow conditions, such as incoming wave amplitude and frequency, on bolus formation and propagation. Additional velocity and density field data will quantify the mass transport of internal bore propagation, as well as quantify the bolus entrainment of ambient fluid.
Overall, the project will strongly complement ongoing field studies examining mass transfer issues in coastal ecosystems. Additionally, the expected results will not only provide insight to the physical mechanisms of shoaling internal waves, but will also have implications for biological processes affected by hydrodynamics and mass transfer in shallow coastal systems. Finally, studies such as these can be invaluable to interpreting the "snapshots" that ocean data often gives, and in perhaps directing further ocean measurements. Results from this work will be of use to researchers studying near-coastal transport and mixing processes in coral reefs, kelp forests, seagrass communities, as well as those concerned with human health impacts from ocean outfalls and other antrhopogenic discharges. Finally, the methodologies and techniques for PLIF and PIV in stratified flows that are further developed and refined here will be of use to the experimental fluid mechanics community.
